1. Field of the Invention
This invention relates to a parameter detection system, an infrared band pass filter, and a glass substrate for the infrared band pass filter as well as a method for detecting parameters.
2. Description of the Related Art
A parameter detection system as described above can for example be used for detecting parameters in individuals, such as iris recognition, 3D scanning, touch sensors, biometrics, interactive displays, gaming and gesture control.
In the sense of this invention a “parameter detection system” is a typically electronic system that is capable of measuring at least one parameter of at least one individual or object. The parameter that is measured can be selected from any parameters that can be measured using optical means. “Detection” includes qualification and/or quantification of the respective parameter.
2. Description of the Related Art
Gesture control devices, iris scanners and other related parameter detection devices are as such known from the prior art. These devices typically comprise an infrared light source for illuminating the area to be detected (“illumination unit”). The wavelength irradiated by the light source typically is in the area of from 800 to 900 nm. In order to capture infrared light that is coming back from the area to be detected, e.g. the person who uses the device, it is preferable that only the wavelength of light which carries the useful information is measured. Measuring only the desired wavelength and filtering out other ranges of the wavelength spectrum, increases the signal-to-noise (S/N) ratio and allows the illuminating light intensity to be decreased. For this purpose infrared band pass filters are used that have good transmission in the desired wavelength regions. The wavelength region that passes the filter is called “passband region”.
The reason why infrared light is used to illuminate the scene is that the S/N ratio can be improved, in particular in environments with high brightness in the visible wavelength range.
Other components that may be used in such a device are a lens that gathers the light reflected from the scene and an image sensor such as a time-of-flight camera. The image sensor measures the time the light has taken to travel from the illumination unit to the detected object and back. Thus, the devices usually comprise an illumination device, a band pass filter, and an image sensor.
WO 2013/010127 A2 teaches biometric imaging devices and methods. The systems described therein contain a light source and an imaging device. An infrared transparent medium can be used to conceal the imaging device from the individual. The infrared transparent medium can be made of glass or plastic and it can include a coating. The document focuses on the semiconductor device used in the imaging device. The imaging device may also include an infrared filter. No further details are discussed.
U.S. Pat. No. 8,750,577 B2 discloses a method and apparatus for eye-scan authentication using a liquid lens. US 2013/0227678 A1 relates to a method and system for authenticating a user of a mobile device. A lot of different configurations for detection systems of biometric data and other parameters have been published. However, little emphasis has been put on optimizing band pass filters for use in such devices.
Different substrates can be used for infrared band pass filters. Each substrate has certain properties and a benefit in one property may be accompanied by a drawback regarding another property of the substrate. Most band pass filters comprise a substrate and one or more coatings. Some of the properties that such a filter should have are the following:                high transmission in the passband region;        very low transmission in the block region;        scratch resistance;        resistance to breakage even at low thickness;        good chemical stability, e.g. hydrolytical stability;        compatible thermal expansion;        optimized optical properties;        low angle dependency of optical properties;        low content of environmentally harmful or toxic components;        low specific weight;        low radiation (fluorescence, phosphorescence, radioactivity);        low manufacturing costs;        availability in low thicknesses and with low thickness variance; and        thermal shock resistance.        
High transmission at the desired wavelength is of particular relevance because light intensity cannot be increased to very high values. Light of very high intensity will damage the user's tissue, in particular in iris recognition systems. Also, illuminating the scene with high light intensities requires a lot of energy.
Many parameter detection systems are useful in portable devices such as mobile phones and tablet or laptop computers. Portable devices are subject to variations in ambient temperature. For example, a mobile device should work not only indoors but also when being used outside. During outdoor activities (such as skiing) very cold temperatures may affect the device. Also, very hot temperatures may occur in situations when a device is in direct sun light. Generally, the temperature range within which a portable electronic device or a device for outdoor use in general should work properly is roughly from −40° C. to 60° C., i.e. in a temperature range of about 100° C. It has been found that this is not a self-evident property in many materials.
It has been found that the optical properties should not only be good for the desired application but the optical properties should also remain as constant as possible over the indicated temperature range. While transmission does usually not vary very much with changing temperature, the refractive index varies to a significant extent with changing temperatures. Even this may not be very problematic in certain optical systems, but when it comes to coated systems, the refractive index change causes a change in transmission as well. This is all the more relevant when it is considered that many parameter detection systems perform very delicate measurements. In iris recognition systems for example the structure of the human iris is detected. In order to work properly the system must be calibrated. A system may be calibrated at room temperature and later on used outside at much lower or higher temperatures. Systems with high temperature dependence of refractive index will suffer from bad parameter detection properties when the device is e.g. calibrated at room temperature and used at substantially different temperatures.